STRUCTURE AND CRYSTAL-CHEMISTRY OF THE HIGH-TC SUPERCONDUCTOR YBA2CU3O7-X
STRUCTURE AND CRYSTAL-CHEMISTRY OF THE HIGH-TC SUPERCONDUCTOR YBA2CU3O7-X
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DOI：
10.1038/327310a0
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发表时间：
1987-05-28
期刊：
影响因子：
64.8
通讯作者：
GRACE, JD
中科院分区：
文献类型：
--
作者：
DAVID, WIF;HARRISON, WTA;GRACE, JD
Y2 O3-BaO-CuO相图中的材料最近引起了极大的兴趣，因为观察到临界温度（Tc）为1.90 K 1，2的超导性;目前的共识是YBa 2Cu 3 O 7 −x（x <$0.2）是超导相（例如参见参考文献3）。Hazenet等人4从单晶X射线衍射实验中得出结论，其结构基于角连接的八面体钙钛矿结构：四方晶胞（空间群P&4bar;m2）来自钇和钡阳离子的有序化以及相关的氧缺陷导致的c轴的三倍。在这里，我们提出了高分辨率的中子粉末衍射数据为这个阶段x = 0.15（7）。我们的结果在阳离子位置方面与参考文献4一致，但是一组氧缺陷的位置是实质上不同的，并且需要将晶格对称性从四方晶系降低到正交晶系（空间群Pmmm），这与单独的发现5 -8一致。与35-K超导体La_（1.85）Ba_（0.15）CuO_4不同，角连接的CuO_4平面基团不仅在a-b平面上以片的形式连接，而且还以平行于ft轴的链的形式连接。铜的平均价态为2.23（5）。从键价参数，我们推断，Cu 2+和Cu 3+离子优先占据四方锥和正方形平面网站分别。晶体化学的考虑表明，这些结构特征可能涉及超导机制。
Materials within the phase diagram Y2O3–BaO–CuO have recently generated great interest because of the observation of superconductivity with a critical temperature (Tc) of ≳90 K1,2; the current consensus is that YBa2Cu3O7−x(x≈ 0.2) is the superconducting phase (see, for example ref. 3). Hazenet al.4concluded, from a single-crystal X-ray diffraction experiment, that its structure is based on the corner-linked octahedral perovskite structure: the tetragonal unit cell (space groupP&4bar;m2) arises from a tripling of the c-axis resulting from the ordering of yttrium and barium cations and associated oxygen defects. Here we present high-resolution neutron powder diffraction data for this phase withx= 0.15(7). Our results agree with ref. 4 with respect to cation positions, but the location of one set of oxygen defects is substantially different and necessitates the lowering of lattice symmetry from tetragonal to orthorhombic (space groupPmmm), in agreement with separate findings5–8. In contrast to the 35-K superconductor, La1.85Ba0.15CuO4, the corner-linked CuO4planar groups are connected not only as sheets in thea–bplane but also as chains parallel to the ft-axis. The average copper valence is 2.23 (5). From bond valence arguments we infer that the Cu2+and Cu3+ions preferentially occupy square pyramidal and square planar sites respectively. Crystal chemical considerations suggest that these structural features may be involved in the superconducting mechanism.